In semiconductor processing, a semi-conducting wafer must be processed in a multiplicity of fabrication steps, i.e., as many as several hundred, in order to complete the manufacturing of an IC device. These processing steps may include etching, cleaning, deposition and various other processing procedures. A variety of chemicals, including liquids and gases must be used in the various processing steps either to etch a specific feature on the IC chip, to clean after certain processing steps, to deposit layers from reactant chemicals, or to carry out other necessary processes.
A variety of specialty chemicals are used for photo and metal cleaning processes. An important requirement for such specialty chemicals, i.e., photoresists, developers, spin-on glass and polyimide is the transporting and storage of the material. In the case of a photoresist material, the photosensitivity and the lifetime of such a material depends on its storage temperature. It is important to maintain such materials within a specification of between 5.degree. C. and 20.degree. C. for a photoresist/developer and between -20.degree. and 10.degree. C. for spin-on glass/polyimide materials.
The transporting of these specialty chemicals, especially liquids, or the delivery from a storage reservoir (i.e., a holding tank) to a processing chamber where the liquid is used is another important aspect of the fabrication process. A process liquid, such as that of a photoresist or a developer, can normally be transported in a fluid passage such as a stainless steel tubing by electrical pump means. One of such conventional liquid delivery system for a photoresist is shown in FIG. 1.
As shown in FIG. 1, the photoresist delivery system 10 generally consists of a liquid reservoir or a holding tank 12, an electrical pump 16, a filter means 18, a needle valve 22, a manual shut-off valve 24 and a drain tank 28. The outlet 32 from the filter means 18 sprays a jet of the photoresist material onto a rotating wafer such that the wafer can be uniformly coated with a thin layer of the photoresist material. The filter means 18 (or a liquid/air separating means) is a device wherein air bubbles trapped in the photoresist solution can be separated from the solution and be released out of the system through needle valve 22. During the process when the photoresist material 14 contained in the holding tank 12 is pumped by the electrical pump 16, air bubbles can be generated and be trapped in the photoresist solution contained in the flow passage 20. When the liquid containing air bubbles passes through the flow passage 20 and enters into the filter means 18, the air bubbles are more likely separated and cumulated to the top of the filter means 18. A jet of the photoresist solution is then released from outlet 32 onto a rotating wafer 26. During normal operations, the needle valve 22 is opened slightly such that the photoresist liquid containing air bubbles can be pushed out of the filter means 18 through passage 30 and needle valve 22 into a waste drain tank 28. The flow of the waste photoresist solution can further be controlled by a manual shut-off valve 24 positioned in between the needle valve 22 and the drain tank 28.
During normal operations, a small amount of air bubbles can be purged out by the above described procedure, i.e, by leaving the needle valve 22 slightly open such that the pressure in the filter means 18 pushes out air bubbles together with a volume of the photoresist solution. However, when a large volume of air bubbles is generated, i.e., during a maintenance procedure of filter replacement or during a photoresist solution change in the holding tank 12, the filter means 18 cannot effectively exhaust the air bubbles unless a large volume of the photoresist solution is purged out and wasted. In other words, the filter means 18 is no longer effective in separating and accumulating the air bubbles when the volume of the bubbles exceeds a critical amount.
When a wafer surface is coated with a photoresist material, the volume of the photoresist material coated and the resulting photoresist film formed must be quantitatively controlled to a high accuracy. Since the presence of air bubbles in the photoresist solution decreases the amount of the photoresist, the amount of the photoresist material available for covering the wafer surface is reduced accordingly. A non-uniform coating and subsequently, a defective pattern can be produced under such circumstances. A poor coating of photoresist film and a poor patterning can result from such a defective coating process due to the presence of air bubbles.
The filter means 18 is a device which holds a filter element (FIGS. 2 and 2A) therein. The filter element is an expendable part of the fabrication equipment and must be replaced after a certain usage time or after a back pressure measured on the filter element reaches a certain maximum value. The replacement normally occurs when a preventive maintenance procedure is performed on the photoresist supply line. Each time the filter element is replaced, considerable effort must be used to pre-wet the filter element. Since the element is normally constructed of pleated sheets of fused fibers and contains air when the element is in a dry state.
Conventionally, a new filter element is placed in a container and soaked in a thinner material. The soaking time is extensive, i.e., at least one hour, in order to ensure a complete wetting of the filter element. After the element is thoroughly wetted by the thinner material, the element is placed in a photoresist supply line in a filter container. A photoresist liquid must then be purged through the line, i.e., the filter element, to drive out the thinner in the element and to completely replace the thinner with the photoresist liquid. Since the thinner material normally has a substantially lower viscosity than the photoresist liquid, any residual thinner in the photoresist would act as a diluent for the photoresist and causing consistency problems in the photoresist coating formed on a wafer. The photoresist liquid purging process further consumes a significant amount of the material, i.e., in a range of about 4/3 liter of the photoresist liquid. The conventional process for pre-wetting a filter element is therefore not only time consuming but also very costly.
It is therefore an object of the present invention to provide a method for wetting a filter element that does not have the drawbacks or shortcomings of the conventional methods.
It is another method for wetting a filter element for use in a semiconductor fabrication process by a high viscosity liquid.
It is a further object of the present invention to provide a method for wetting a filter element for use in a semiconductor fabrication process by a high viscosity liquid that does not require the use of a thinner material to pre-soak the element.
It is another further object of the present invention to provide a method for wetting a filter element by a high viscosity liquid by utilizing a sealable tank adapted for holding a pressurized high viscosity liquid.
It is still another object of the present invention to provide a method for wetting a filter element with a high viscosity material by using a sealable container that is equipped with a vibration device for vibrating the element soaked in the liquid.
It is yet another object of the present invention to provide a method for wetting a filter element with a high viscosity liquid by placing the element in a sealable container equipped with a vibration device such that trapped air bubbles in the element may be separated from the element.
It is still another further object of the present invention for pre-wetting a filter element with a high viscosity liquid by pressurizing a high viscosity liquid to a pressure higher than 1 atm and vibrating the container at an ultrasonic frequency.
It is yet another further object of the present invention to provide an apparatus for wetting a filter element that includes a sealable container adapted for holding a pressurized liquid and a filter element wherein a vibration device is mounted on the container for generating a vibration in the liquid such that trapped air bubbles are separated from the filter element.